1. Field of the Invention
The present invention relates to an output voltage regulator for an engine-driven generator, and more particularly, to an output voltage regulator for an engine-driven generator that can suppress fluctuation in output voltage, even when an output waveform of the generator is distorted under the influence of a load, by accurately detecting a stable alternating current cycle and providing feedback control at an effective value of voltage.
2. Description of the Related Art
As a means for suppressing fluctuation in output voltage of a generator, for example, an automatic voltage regulator disclosed in Japanese Published Examined Patent Application No. H03-41038 is known. FIG. 3 is a circuit diagram according to an example of an automatic voltage regulator conventionally known. The automatic voltage regulator is provided with an excitation winding 11, a control circuit 12, a field winding 13, a detection winding 14, and a detection circuit 19. The detection winding 14 is connected in series to an unillustrated output winding. Output voltage of a generator is detected as a peak voltage of a ripple voltage by the detection circuit 19 composed of the detection winding 14, a full-wave rectifier 15, a capacitor 16, and resistors 17 and 18. The control circuit 12 has a Zener diode 20 and Darlington-connected transistors Q1 and Q2, and controls a field current “if” according to a voltage Vc obtained by dividing the ripple voltage by the resistors 17 and 18. Namely, when a relationship between the voltage Vc and a set voltage Vz by the Zener diode 20 is Vc<Vz, the transistor Q1 is turned off and the transistor Q2 is turned on. As a result, to the field winding 13, output voltage of the excitation winding 11 flows as the field current “if”.
FIGS. 4A-4C are views showing operations of the circuit shown in FIG. 3. When a load connected to a generator is light, the section with Vc<Vz is narrow and the field current “if” is small as shown in FIG. 4A. On the other hand, when the load increases, the peak value Vc is lowered to expand the section with Vc<Vz, so that the field current “if” increases.
On the other hand, for example, in such a case that the output waveform is distorted by overloading, the section with Vc<Vz changes according to the distortion in the output waveform as shown in FIG. 4C, and the field current “if” always fluctuates.
The automatic voltage regulator has widely spread in use since the method for detecting and controlling output voltage is simple. However, as described above, since a detection signal obtained by comparison of the peak value Vc of a detection waveform being a ripple current with the set voltage Vz is fed back, there is a problem such that fluctuation in output voltage also increases due to distortion in the output waveform.
A variety of countermeasures have been taken against various phenomena caused by the problem, however, on the assumption that a peak value of the detection waveform is detected, influence of distortion in the output waveform is unavoidable.
For this reason, it has been considered to calculate effective value data by sampling one period of the output waveform as digital data and feedback the effective value data. According to the digital sampling, highly accurate feedback control can be carried out even when distortion in the waveform occurs.
By the control method by digital data sampling, it is easy to judge one period if the output frequency has been fixed at a predetermined frequency set accurately by an inverter, as in an inverter-controlled generator distributed in recent years. However, as in a single-phase synchronous generator driven by an engine, in a generator for which an output frequency is determined based on the number of engine revolutions and the number of magnetic poles, since it is necessary to distinguish one period based on a zero-cross point of the output waveform, one period can be erroneously detected due to distortion in the output waveform occurs. For example, in such a case of capacitive and inductive loads as in a mercury lamp or an electric power tool, a zero-cross point sometimes occurs multiple times in the interval of one period.